the perimeter sensor you are referring to is like the proximity sensor. it emits a field around the car and sets off a warning if someone gets too close. if they get closer than that then the alarm will of off completely. the microwave interior acts kind of the same way. it is usually used in cars that have convertibles. it emits a field inside the car and when there is an opening, like for example some fool decides to cut the convertible to try to get in the car, then the field will naturally become larger thus setting off the alarm. basically, when you set it, it will create a field inside the car, and if some makes a hole, like cutting the convertible or breaking the glass then the alarm will go off.
the proximity or the perimeter sensor is ideal for any car because for example, someone can break your glass and run back to their car and wait for the alarm to go off automatically. then, all they have to do is climb in the window gently and take what they want. with the proximity sensor, they can't climb in the window because the alam will always go off no matter what because you are disturbing the field.